Time to boil a water kettle

My friend asks me this question, I have not worked it out.
I hope it is suitable to maths box .

The time to boil a small kettle of water using a electric coil is A minutes.
Q: How many minutes needed to boil a big kettle (two times the mass of water compared to the small kettle), if using the same coil. Consider no heat is transmitted away to the ambient.